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On 2 July 1890 the Rhondda Tunnel, a distance of 3,443 yards was completed for the Rhondda and Swansea Bay Railway.
The sponsors of the Rhondda and Swansea Bay Railway were keen to link Swansea and it's docks with the booming Rhondda coalfield. Aside from the obvious business benefit, it was argued that Swansea was nearer the sea than Cardiff, which mattered of course because of the massive export of Welsh coal.
The single-line tunnel split into double track on emerging from Rhondda reaching Blaen Rhondda, the RSBR's only station in the Rhondda. From there, the line continued to the Taff Vale Railway at Treherbert.
In attempt to cross the estuary of the river Neath at Briton Ferry, Sydney William Yockney began to build a tunnel, but it was abandoned due to flooding. Some of the earthworks on the Llandarcy/Swansea side were still visible until the M4 flyover was built. Pre M4 OS maps clearly show the earth works. He subsequently built the swing bridge up stream. It has been claimed that this was the only swing bridge in the UK (world?) built on a curve, and in any event a swing bridge on a curve is extremely rare. This bridge still exists.
Built and engineered by Sydney William Yockney (a pupil of Brunel), construction of the Rhondda Tunnel was carried out from both ends, taking five years to bore from June 1885 to 2nd July 1890. It features a single 58-foot ventilation shaft around 105 yards from its western end. It's almost 1,000 feet below ground at its deepest point.
Normally in the construction of a tunnel, in addition to the headings at each end, shafts are dropped at various locations to allow work within the tunnel; thus there are a number of headings at work together. Due to the height of the mountain it was not possible to drop shafts at various locations to allow work within the tunnel therefore there were only two headings at each end, one from the Afan Valley and the other from the Rhondda. When they eventually met, they measurements were out - by half an inch.